Underwater broadband acoustic transducer

ABSTRACT

An underwater acoustic transducer configured to emit a low-frequency broadband acoustic signal. The underwater acoustic transducer includes a cap in the form of a cylinder closed on one side and open on the other side. A piezoelectric plate operating in flexion is fixed on the base of the cylinder outside the cylinder. The inside cavity defined by the cylinder is open freely toward the outside medium in which the acoustic waves are to be emitted. For identical dimensions, the underwater acoustic transducer makes it possible to decrease a central frequency of emission and to increase bandwidth.

The present invention relates to broadband underwater acoustictransducers which are essentially used as acoustic signal emitters inthe active sonars of surface vessels. However, these transducers mayalso be used as receivers, and also in sonars for submarines.

It is known that with progress in sonars, one seeks to decrease theworking frequency, so as among other things to increase the range ofthese sonars, and the emission power. One also seeks to have transducerswhich operate in broadband so as to be able, through suitableprocessing, to circumvent the phenomena of reverberation, and to be ableto use several sonars in one and the same geographical zone, therebyachieving interoperability of systems.

At present the most commonly used emitter transducers are of theso-called “tonpilz” type. These transducers use an emitter horn which isexcited by a ceramic pillar which bears on a countermass.

These “tonpilz” emitters make it possible to obtain a good level ofemission and considerable back rejection, allowing channel formationwith a low level of secondaries. Furthermore, they are usable both foremission and for reception.

On the other hand they exhibit the disadvantage of having a relativelysmall bandwidth, typically corresponding to a quality factor Q≅3.5.Moreover the transducer/casing join is relatively fragile, giving riseto a risk of water ingress at this level. Finally, since the frequencieswhich can be emitted are strictly related to the dimensions of the horn,it is only possible to drop down in frequency by increasing thesedimensions, this rapidly becoming prohibitive.

To alleviate these drawbacks, the invention proposes a broadbandunderwater acoustic transducer, comprising at least one piezoelectricplate operating in flexion, principally characterized in that itfurthermore comprises a cylindrical cap closed at one end by a baseboardand open at the other end so as to form a first cavity; thepiezoelectric plate being fixed on the outside face of the baseboard andthe first cavity of the cap being open freely toward the outside mediumin which the cap is immersed.

According to another characteristic, the cross section of the cap iscircular.

According to another characteristic, the cross section of the cap iselliptical.

According to another characteristic, the first cavity is filled at leastpartially with a matching material whose acoustic characteristics aredifferent from those of the outside medium in which the transducer isimmersed.

According to another characteristic it furthermore comprises a bodyincluding a second inside cavity closed by the cap in such a way as tobe insulated from the outside medium with the piezoelectric plateenclosed in the second inside cavity and the first cavity pointingoutward.

According to another characteristic, it comprises two cap/piezoelectricplate assemblies fixed together head-to-tail.

According to another characteristic, the cap forms the front face of atransducer of the so-called “tonpilz” type.

Other features and advantages of the invention will become clearlyapparent in the following description, presented by way of nonlimitingexample with regard to the appended figures which represent:

FIG. 1, a sectional view of a transducer according to the invention;

FIG. 2, a chart of frequency/amplitude of emission of such a transducer;and

FIG. 3, a longitudinal sectional view of a variant of the invention, inwhich the transducer is dual.

The device according to the invention represented as a longitudinalsection in FIG. 1 exhibits a structure which, as may readily beobserved, is a complete break with the technology used currently, whichrelies on the above-cited “tonpilz” structure.

This device comprises as active element a ceramic board 101, preferablya single such board, and which in a conventional manner comprises a pairof electrodes 102 and 103 each fixed on one of the main faces of thisboard. Preferably these electrodes are constructed by silver plating.These electrodes are linked by wires 104 to an amplifier which deliversan excitation signal at the desired frequency. Given the structure ofthe apparatus, it would be entirely possible to limit these supply wiresto a single wire linked to the electrode 103 which is insulated. Theother electrode, which is linked to the earth of the apparatus, wouldthen be supplied by way of this earth.

The electrode 102 is fixed on the lower plane face of a member 105 inthe form of a cylinder closed at its base and open at its upper end. Weshall refer to this member as a “cap”.

The vibrations of the ceramic board 101 are transmitted to the cap,whose structure starts vibrating according to two main modes ofresonance. The critical couplings of these two modes of resonance thenmake it possible to obtain a large bandwidth, corresponding to around60% of the central frequency.

The first mode of resonance is the natural mode of flexion of the lowerface of the cap under the action of the ceramic working in mode 3.1.

The second mode originates from the action of the fluid filling theinside cavity 106 formed by the cap which is immersed directly in theoutside medium, seawater in general. Indeed in this cavity the speed ofthe acoustic waves is lower than in free space, since the walls of thecap are not infinitely rigid. A mode of resonance corresponding to a λ/4plate is then obtained. The more the rigidity of the walls increases,the more the top frequency increases. The more the height of the wallsincreases, the more the bottom frequency decreases.

As represented in the figure, the invention also proposes that the lowerwall 107 of the cap should exhibit a central thickening such that thecross section of this plate corresponds to the shape of a beam of equalstrength. In this way, the constraints applied by the pressure of theoutside fluid on the ceramic plate 101 by way of the bottom 107 of thecap are uniformly distributed over this plate, thereby preventing itfrom curving under the action of this pressure and hence thuseliminating the risks of breakage of the ceramic plate under the effectof the pressure.

This shape increases the area of radiation into the fluid by a factor of2. In total one thus obtains better efficacy of the ceramic, bettermechanoacoustic efficiency and a reduction in the cavitation thresholdas compared with a standard flexion transducer.

In the embodiment represented in the figure, the transducer issupplemented with a body, or “tape”, 107 which has the shape of acylinder concentric with the cap 105 and which at its upper partexhibits a cavity 108 into which the cap will be engaged. This cap isfixed by its outside lateral face to the body by welding for example atthe level of the upper end of this body. This fixing 109 is in the formof a thickening inside the cavity 108 in such a way as to leave a freespace 110 between the internal wall of the cavity 108 and the externalwall of the cap 105, so as to avoid disturbing the vibratory regime. Thesupply cables 104 exit the body via an axial channel 111 which emergeson one side in the cavity 108 and on another side on the lower surfaceof the body. This axial channel is plugged by means (not represented), ascrew-type plug for example, which make it possible both to ensure theconnection of the wires 104 and the hermetic sealing of the cavity108/111. In this way, this cavity remains filled with air without theoutside water penetrating into it, which allows the ceramic plate 101 tovibrate and would also short-circuit the electrodes 101.

According to an exemplary embodiment, represented in FIG. 2 is a curveof sensitivity to emission for such a transducer whose cap 105 exhibitsan outside diameter of 115 millimeters with a thickness of the lateralwalls of 4 millimeters, and a total height of 46 millimeters with acentral thickness of the lower face of the cap equal to 14 millimeters.The broadening of the frequency band is clearly observable in thiscurve. Furthermore, this frequency band is shifted toward the lowfrequencies for a dimensioning which would corresponds for aconventional transducer of the “tonpilz” type to a markedly higheremission frequency.

By way of variant, the invention also proposes that the cap 105 be madein the shape of a cylinder with an elliptical rather than circular crosssection. This then makes it possible to obtain two distinct resonancesat the level of the cavity 106, in addition to the resonance of theceramic plate 101. In this way the bandwidth is further increased.

It is also possible to use the cap 105/ceramic plate 101 assembly on itsown, without appending the body 107 thereto, but while still ensuringthe insulation of the electrodes 102 and 103 by an appropriate coating,a layer of waterproof paint for example. This transducer, which is thenof the so-called “free flooded” type, can be used without any limit ofsubmersion but however with a lower efficiency due to the action of thewater on the back face of the ceramic. In this case the height of thecavity will advantageously be chosen to be equal to half the centralwavelength of the transducer, so as to obtain good matching whileperforming a rephasing between the waves emitted forwards and thosewhich in this case are emitted backwards.

The invention also proposes, by way of variant, that the cavity of thecap 105 be filled, possibly to a height which is not equal to that ofthe cap, with a matching material whose acoustic characteristics, inparticular the speed of propagation of sound, are different from thoseof water. This makes it possible to modify the response curve, forexample to make it flatter or to broaden it even more.

Another variant, represented in FIG. 3, consists in using twocap/ceramic assemblies, one 105/101 and the other 205/201, fixedhead-to-tail on a cap 117 exhibiting the shape of a cylinder open onboth sides. The connecting wires 114 to the two ceramic plates then exitvia a connector 211 fixed on the lateral wall of the body 117. Such anarrangement makes it possible to obtain a transducer exhibiting aradiation of dipolar type, characterized by considerable rejection alongits longitudinal axis.

Finally, an extension of the invention consists in contriving theemission horn of a known transducer of “tonpilz” type, in such a waythat it takes the shape of the cap 105. A broadband “tonpilz” transduceris thus obtained which, by comparison with the basic embodiment of FIG.1, makes it possible to obtain a greater emission power by virtue of theceramic stack characteristic of a “tonpilz”. However this advantage isachieved at the cost of an increase in bulkiness and a return to theknown leakproofing problems of the “tonpilz” system since it is thennecessary to maintain the freedom of clearance of the horn of the“tonpilz” with respect to the body of the latter.

To summarize, the invention makes it possible, relative to thetechnology currently used, to simultaneously obtain a broadening of thefrequency band emitted, a shifting of this band toward the lowfrequencies without modifying the bulkiness of the device, animprovement in the watertightness of the front face, and a reduction inthe cost of the apparatus by decreasing the number of members used forits manufacture.

What is claimed is:
 1. Broadband underwater acoustic transducer,comprising: at least one piezoelectric plate operating in flexion; abaseboard having a first predetermined mode of resonance, said baseboardhaving a variable thickness; and a cylindrical cap closed at a first endby the baseboard and open at a second end to form a first cavity, saidcap having a second predetermined mode of resonance different than saidfirst predetermined mode of resonance, wherein the at least onepiezoelectric plate is fixed on an outside face of the baseboard and thefirst cavity is open freely toward an outside medium in which the cap isimmersed.
 2. The transducer as claimed in claim 1, wherein across-section of the cap is circular.
 3. The transducer as claimed inclaim 1, wherein a cross-section of the cap is elliptical and said caphas a third predetermined mode of resonance.
 4. The transducer asclaimed in claim 1, wherein the first cavity is filled at leastpartially with a matching material having acoustic characteristicsdifferent from those of the outside medium in which the transducer isimmersed.
 5. The transducer as claimed in claim 1, further comprising abody including a second inside cavity closed by the cap to be insulatedfrom the outside medium with the at least one piezoelectric plateenclosed in the second inside cavity and the first cavity pointingoutward.
 6. The transducer as claimed in claim 1, further comprising twocap/piezoelectric plate assemblies fixed together head-to-tail.
 7. Thetransducer as claimed in claim 1, wherein the cap forms a front face ofa tonpilz transducer.
 8. The transducer as claimed in claim 1, whereinsaid baseboard has a central thickness greater than a peripheralthickness.